The present invention relates to a method and apparatus for conditioning a polishing pad. More particularly, the present invention relates to a method and apparatus for conditioning a fixed-abrasive polishing pad used in the chemical mechanical planarization of semiconductor wafers.
Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. A common technique for forming the circuitry on a semiconductor wafer is photolithography. Part of the photolithography process requires that a special camera focus on the wafer to project an image of the circuit on the wafer. The ability of the camera to focus on the surface of the wafer is often adversely affected by inconsistencies or unevenness in the wafer surface. This sensitivity is accentuated with the current drive for smaller, more highly integrated circuit designs which cannot tolerate certain nonuniformities within a particular die or between a plurality of dies on a wafer. Because semiconductor circuits on wafers are commonly constructed in layers, where a portion of a circuit is created on a first layer and conductive vias connect it to a portion of the circuit on the next layer, each layer can add or create nonuniformity on the wafer that must be smoothed out before generating the next layer.
Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of material added thereafter. Available CMP systems, commonly called wafer polishers, often use a rotating wafer holder that brings the wafer into contact with a polishing pad moving in the plane of the wafer surface to be planarized. In some systems, a polishing fluid, such as a chemical polishing agent or slurry containing microabrasives, is applied to the polishing pad to polish the wafer. The wafer holder then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer. In other CMP systems, a fixed-abrasive polishing pad is used to polish the wafer. In fixed-abrasive applications, the wafer holder presses the wafer against the rotating fixed-abrasive polishing pad, deionized water (or some other non-abrasive substance) is applied, and the pad is rotated to polish and planarize the wafer. Some available wafer polishers use a linear belt rather than a rotating surface to carry the polishing pad.
With use, the polishing pads used in standard, abrasive/chemical slurry CMP systems become smoothed and clogged with used slurry and debris from the polishing process. The accumulation of debris reduces the surface roughness and adversely affects polishing rate and uniformity. Polishing pads are typically conditioned to roughen the pad surface, provide microchannels for slurry transport, and remove debris or byproducts generated during the CMP process. Standard methods for conditioning this type of polishing pad may use a rotary disk embedded with diamond particles to roughen the surface of the polishing pad.
CMP systems using a fixed-abrasive pad generally require the presence of features on the semiconductor wafer to function. One type of fixed-abrasive pad includes abrasive particles embedded within a polymer matrix. With this type of fixed-abrasive polishing pad, a patterned semiconductor wafer conditions the fixed-abrasive pad as it is polished by using the topography features created by the etching and deposition processes on the semiconductor wafer to remove a portion of the polymer matrix, thus exposing the abrasive particles embedded within. In fact, a patterned semiconductor wafer is sometimes used to pre-condition a pad. These wafers, sometimes referred to as dummy patterned wafers, can cost a considerable amount of money to manufacture, and the loading of these dummy wafers onto a CMP system may take up a considerable amount of time.
Another difficulty encountered with fixed-abrasive pads is the uneven polishing rate and polishing uniformity that can develop on a wafer-to wafer basis. Because a circular wafer presents more length of surface with topography in the center than at the edges, the wafer tends to expose abrasives on the pad surface at an uneven rate from the center to the edge of the wafer. Thus, an inconsistent polish result can occur, and become more pronounced, as a fixed abrasive polishing pad is used with consecutive wafers. One way to condition the fixed-abrasive pad is to use a standard abrasive-type conditioner to try and attain a desired amount of exposed abrasive. A drawback of this method is the difficulty in controlling the resulting roughness. Another possible method for improving polishing pad performance is to fabricate a fixed-abrasive polishing pad having an abrasive matrix that is different in the center than at the edges in order to compensate for the center-to-edge effects. A drawback of this technique is that such a variable abrasive pad may be difficult and expensive to produce.
Accordingly, further development of an apparatus and method for conditioning a fixed-abrasive pad used in the chemical mechanical planarization of semiconductor wafers is necessary in order to decrease the cost and time for conditioning a fixed-abrasive pad.